1. Field of the Invention
The present invention relates to a color zoom copying apparatus, and more precisely relates to a color separating device therefor.
2. Description of Related Art
In a conventional color zoom copier, a zoom lens is used as an imaging optical system, and image data of an object are separated into three colors, i.e., R (Red, 620 nm), G (Green, 525 nm), and B (Blue, 450 nm) to successively make latent images on a photoconductive drum. These latent images are developed with yellow, magenta, and cyanine and are superimposed to produce a color copy.
FIG. 7 shows a copying system of a known color zoom copier. In FIG. 7, a scanning unit S having therein an illuminating light source 12 and scanning mirrors 13 and 14 is provided below a transparent glass plate 11 on which an object (document) 0 is located. The scanning unit S having the illuminating light source 12 and the scanning mirrors 13, 14 incorporated therein is scanned from a position shown by a solid line S to a position shown by an imaginary line S'(12', 13' 14'). The light which is emitted from the light source 12 (12') is reflected by the document 0 and then by the mirrors 13 and 14 and is transmitted onto a photoconductive drum 17 through a wavelength selecting filter (color separating mechanism) 15, an imaging optical system 16, and immovable mirrors M.sub.1, M.sub.2, M.sub.3. On the circumference of the photoconductive drum 17 are provided various known color copying elements, such as a charger 18, a developing unit assembly 19 having developing units (19a for yellow, 19b for magenta, 19c for cyanine and 19d for black) corresponding to the respective selected wavelengths, and a transfer unit 20, etc. In FIG. 7, numeral 21 designates a paper on which the image is to be copied, and 22 a paper feeder therefor.
The zoom lens system 16 has a front group of movable lenses (which will be referred to as a front lens group L.sub.1) and a rear group of movable lenses (which will be referred to as a rear lens group L.sub.2). The zoom lens system 16 varies the spatial distance between the front lens group L.sub.1 and the rear lens group L.sub.2 and moves to vary the imaging magnification (i.e., the copy magnification) without changing the distance between the object and the image. FIG. 9 shows typical tracks of movement of the lens groups L.sub.1 and L.sub.2 in accordance with the variation of magnification, in a known device.
In the known arrangement shown in FIG. 7, the color separating mechanism 15 which is located in front of the zoom lens system 16 successively inserts color filters of the three (or four) colors R, G and B (and M for monochrome, if necessary) in the optical path of the zoom lens system to effect color separation. Alternatively, it is also known to arrange, between the light source 12 and the document 0, the color separating mechanism in which the color filters R, G and B are selectively inserted in the optical path to carry out color separation. When the scanning unit S is scanned for the respective colors, image data (i.e., the latent image) which are essentially separated into three colors R, G and B are formed on the photoconductive drum 17.
In the developing unit assembly 19, the developing unit (yellow) 19a is used for the latent image which is formed by the color filter B, the developing unit 19b (magenta) for the latent image which is formed by the color filter of G, and the developing unit 19c (cyanine) for the latent image which is formed by the color filter R. The latent images developed on the photoconductive drum 17 are superimposed on the same paper 21 to obtain a desired color copy.
In the color zoom copier, when color separation is effected and the magnification is changed, as mentioned above, an image is out of focus due to the chromatic aberration of the lenses in accordance with the color and the copy magnification. FIG. 8 shows examples of positions of focal points of the zoom lens system when the color filters G, B and R are inserted in the optical path, and a deviation in focus (i.e., amount of defocus), in accordance with variation in the magnification.
It is apparent that the deviation decreases the quality of the image and, accordingly, it is desirable to control the zoom lens system 16, so that the focal points are always located on the photoconductive drum 17.